1. Field of the Invention
The present invention relates to a mobile communications system for conducting uninterrupted hard handover.
2. Related Art
As disclosed in application no. 2001-309432 filed in Japan, W-CDMA (wideband code division multiple access) allows a single mobile terminal to conduct simultaneous communication of a plurality of calls grouped into audio communications, packet communications, and combined audio/packet communications. An example of this is shown in FIG. 1. FIG. 1 shows a mobile terminal 1 conducting audio communication with another mobile terminal via a base station 2 and a base station 3, while at the same time conducting packet communication with a server 5 over the Internet.
Furthermore, as disclosed in application no. 2001-231063 filed in Japan, technology that allows for the switching of channels without interruption, referred to as uninterrupted hard handover (hereafter “HHO”), has become standardized in W-CDMA.
In a mobile communications system that uses these technologies, it is possible to vary the number of calls during communication without interruption. Thus, for example, if a mobile terminal is conducting audio communication with another mobile terminal using a certain channel, the mobile terminal can conduct HHO during the audio communication and switch to a channel that combines audio and packet communications.
However, the rate at which call blocking occurs is higher when channel switching to add calls then it is when channel switching to reduce calls. This is because the symbol rate and the spreading rate are inversely proportional at a fixed chip rate according to W-CDMA, and thus when a call is added, the symbol rate is increased and the spreading rate is decreased. Here, the lower the spreading rate, the more difficult it is to establish spreading code synchronization.
When call blocking occurs in this instance, not only is it impossible to initiate communication of the additional call, but communication of the connected call is also cut.